This invention relates to improved drive shafts. More particularly, this invention relates to lightweight drive shafts having noise and vibration dampening characteristics.
The function of a drive shaft is to convey torque from a power source to a means for converting this energy into useful work. In the case of conventional automobiles, and other vehicles, the drive shaft conveys torque from the transmission to the differential where it is converted into the drive force for the rear wheels of the vehicle.
Conventionally, automotive drive shafts are constructed of steel, or similar dense material, and have a diameter and thickness which enables them to meet the required torque carrying capacity. However, not only are such conventional shafts undesirably heavy, but, in addition, they have a high longitudinal stiffness, e.g., a longitudinal Young's modulus of elasticity of about 30 .times. 10.sup.6 psi. The excessive weight of such shafts significantly increases the cost of producing and running the vehicle, while the high stiffness of these shafts causes lateral vibrations originating in the engine and transmission to be readily transmitted to the differential when the engine is operated at high speed, resulting in considerable noise in the bearings of the differential. This noise problem is particularly prevalent in smaller cars having smaller engines which are constructed to provide a greater number of crankshaft revolutions per minute than larger cars with larger engines. With the accelerating trend toward smaller cars and energy conservation, this problem has increased greatly in importance. While the problem of vibrations transmitted to the differential by the drive shaft can be overcome to some degree by the use of universal joints within the shaft, this solution adds additional weight to the car, or other vehicle, and further increases the cost and expense of producing and running the vehicle.
Another problem associated with conventional automotive drive shafts is that they sometimes go out of round when rotated, i.e., they undergo diametric distortion, causing a noise described by the automotive industry as high speed "boom". The tendency to go out of round can be reduced by increasing the thickness of the walls of the shaft, but once again only at the expense of adding further weight and cost to the vehicle. In addition, the greater wall thickness further increases the longitudinal stiffness of the shaft, and this magnifies the noise problem in the bearings of the differential.
Drive shafts made from composites of filamentary material, such as boron or carbon fibers, embedded in a solid resin matrix have also been proposed as the rotary means for helicopter blades. In all such shafts, a majority of the fibers employed have always been disposed parallel to the axis of the shaft in order to impart maximum stiffness to the shaft to enable it to resist the high lateral forces to which such shafts are subjected, and none of these shafts have been disigned to overcome the noise and vibration problems associated with the transfer of torque from the engine and transmission to the differential in high speed small engine cars.